When drilling for oil and gas, it is desirable to maintain maximum control over the drilling operation, notwithstanding that the drilling operation might be several kilometers below the Earth's surface. Steerable drill bits, which can achieve directional drilling, are in widespread use, and are often required to drill complex borehole trajectories requiring accurate control of the path of the drill bit during the drilling operation.
Directional drilling is complicated by the necessity to operate the steerable drill bit within harsh borehole conditions. The steering mechanism is typically disposed near the drill bit. In order to obtain the desired real-time directional control, it is preferred to operate the steering mechanism remotely from the surface of the Earth. Furthermore, the steering mechanism must be operated to maintain the desired path and direction regardless of its depth within the borehole and whilst maintaining practical drilling speeds. Finally, the steering mechanism must reliably operate under exceptional heat, pressure and vibration conditions that will typically be encountered during the drilling operation.
Many types of steering mechanism are known: A common type of steering mechanism comprises a motor disposed in a housing with a longitudinal axis which is offset, tilted or otherwise displaced from the axis of the borehole. The motor can be of a variety of types including electric and hydraulic. Hydraulic motors which operate by way of the circulating drilling fluid are commonly known as a “mud” motors. The drill bit is attached to the output shaft of the motor, and is rotated by the action of the motor. The axially offset motor housing, commonly referred to as a bent subsection or “bent sub”, provides axial displacement that can be used to change the trajectory of the borehole. By rotating the drill bit with the motor and simultaneously rotating the motor housing with the drill string, the orientation of the housing offset continuously changes and the path of the advancing borehole is maintained substantially parallel to the axis of the drill string. By rotating the drill bit with the motor only, the path of the borehole is deviated from the axis of the non-rotating drill string in the direction of the offset of the bent sub. By alternating these two methodologies of drill bit rotation, the path of the borehole can be controlled. A more detailed description of directional drilling using the bent sub concept is disclosed in U.S. Pat. Nos. 3,260,318, and 3,841,420.
UK patent applications 2 435 060 and 2 440 024 also describe methods of steering a drill bit by way of the bent housing of a downhole motor. The drill string is rotating and there is a rotatable connection between the drill string and the housing of the downhole motor. A clutch mechanism is provided within the rotatable connection, the clutch mechanism controlling the orientation of the housing and consequently the orientation of the bend.
Another method for steering a drill bit is to utilise a steering mechanism such as that described in our published European patent 1 024 245. That steering mechanism allows the drill bit to be moved in any chosen direction, i.e. the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and as a result of the measured drilling conditions at a particular borehole depth. Another mechanism which can cause a (variable) lateral offset, and thereby deviate the drill bit in a desired direction, and by a desired amount, is disclosed in U.S. Pat. No. 4,416,339.
Directional drilling applications require the drill string, or parts of the downhole assembly, to articulate and/or be flexible so as to pass along the curved borehole.
U.S. Pat. No. 7,766,098 describes a mechanism and method for steering a drill bit by periodically varying the rotational rate of the drill bit. This patent takes advantage of the fact that the rate at which the drill bit removes borehole material is dependent upon its rate of rotation. By varying the rate of rotation of the drill bit cyclically during each 360° rotation of the drill string, the drill bit can be caused to remove more material from one side of the borehole than the other, whereby to cause the drill bit to deviate from a linear path. The cyclical operation of a steering mechanism is a feature of many steering tools used with rotating drill strings, including that of the aforementioned EP 1 024 245.